1. Field of the Invention
The present invention relates to disproportionation of olefins, and, more particularly, to a novel catalyst system which is useful for disproportionation of olefins and to a process for disproportionation of olefins using such a catalyst.
2. Description of the Prior Art
Disproportionation (sometimes called metathesis) of olefins is defined as a reaction in which one or more olefins are converted into other olefins by the exchange of alkylidene groups as shown in the following formula: EQU (R.sup.1)(R.sub.2)C.dbd.C(R.sup.3)(R.sup.4)+(R.sup.5)(R.sup.6)C.dbd.C (R.sup.7)(R.sup.8).fwdarw.(R.sup.1)(R.sup.2)C.dbd.C(R.sup.5)(R.sup.6)+(R.s up.3)(R.sup.4)C.dbd.C(R.sup.7)(R.sup.8)
(In the formula, R.sup.1, R.sup.2, R.sup.3, R.sup.4, R.sup.5, R.sup.6, R.sup.7 and R.sup.8 are independently selected from the group consisting of a hydrogen atom and an alkyl group.)
This reaction was first discovered by Banks and Bailey in 1964 in a reaction in which ethylene and 2-butene are produced from propylene in the presence of a molybdenum catalyst. The discovery has led to successive developments of various disproportionation catalysts, including molybdenum-, tungsten-, and rhenium-type catalysts. However, several problems still remain to be solved concerning with the application of the disproportionation process to the manufacture of industrially useful products with a high purity at a low cost. Some problems which are to be solved are (1) promotion of the selectivity by suppressing isomerization and polymerization reactions, (2) reduction of the induction period which results improvement in the conversion rate and shortening of start-up time, (3) prolongation of the catalyst life by suppressing loss or denaturation of active sites of catalysts which is caused by changes in the valency of metals due to water, dienes or oxygen-containing substances contained in raw materials or in by-products, or by suppressing accumulation of carbonaceous materials contained in by-products onto the catalysts, and the like.
A number of catalyst systems and reaction methods have been proposed in efforts for overcoming these problems. As regards to the above-described problem (1), for example, a method of adding an alkaline metal oxide to a disproportionation catalyst, e.g., to tungsten oxide catalyst carried by silica gel, was disclosed in U.S. Pat. No. 3,579,602 and U.S. Pat. No. 3,586,731; and a method of using a mixed catalyst system in which magnesium oxide or the like is combined with a disproportionation catalyst was disclosed in U.S. Pat. No. 3,707,579. Other catalyst systems combining these catalysts, e.g., MgO+WO.sub.3 /SiO.sub.2 (KOH), were proposed in U.S. Pat. No. 3,915,897, U.S. Pat. No. 3,760,026 and U.S. Pat. No. 4,575,575.
As regards to the problem (2) above, a brief explanation is given below. In the disproportionation reaction of olefins, long induction period results a poor conversion rate. The reason therefor is that during the induction period, i.e., prior to the commencement of disproportionation reaction, poisonous reaction by-products attack the active sites of the catalyst, and carbons and resinous polymeric materials accumulate onto the surface or active sites of the catalyst which markedly impair the catalyst activity even after the completion of the induction period. In the disproportionation reaction of olefins, it is necessary that the valency of tungsten in the active catalyst should be kept in a specific range, and therefore, the existence of induction period is inevitable due to the time required for changing the valency of tungsten contained in the catalyst to the specific range. Method heretofore proposed for overcoming the above-described problem (2) by subjecting the catalyst to a reducing treatment by hydrogen or carbon monoxide prior to use is well known.
With respect to the problem (3), U.S. Pat. No. 3,707,579 disclosed a method of extending the catalyst life by passing hydrocarbon feed stocks through a pre-activated magnesium oxide bed (a guard bed) to remove poisonous components such as water, dienes, oxygen-containing compounds, and the like contained in the starting raw materials.
None of these methods, however, cannot solve all the above-mentioned problems at the same time. For example, the method disclosed in the above U.S. Pat. No. 4,575,575 which utilizes a mixed catalyst system comprising a disproportionation catalyst in which an alkaline metal oxide is combined with magnesium oxide or the like, is insufficient for industrial use because of its short catalyst life due to a rapid decrease in the conversion rate over time, even though it exhibits a significant effect on the promotion of the selectivity.